The invention relates to a carburation device for an internal combustion engine of the type which comprises, in its intake passage, upstream of an operator actuated main throttle member, an auxiliary throttle member which opens automatically and progressively as the air flow passing through said passage increases and which controls a mixture control device metering the fuel flow drawn into a discharge orifice in the passage from a float chamber, said mixture control device comprising a solenoid or electromagnetic valve placed between the float chamber and the orifice and controlled by periodic electrical pulses supplied by an electronic circuit which receives an input signal from an electric component, such as potentiometer, controlled by the movements of the auxiliary throttle member so that the fuel flow per unit of time depends on the position of the auxiliary throttle member.
Carburation devices of the above type are described in U.K Specification Nos. 1 505 438 and 1 505 439, wherein the discharge orifice is placed downstream of the main throttle member and is therefore subjected to a degree of vacuum which varies in a large range during operation of the engine, which renders correction means necessary.
On the other hand, carburettors of the so-called "constant depression" type are known which have a fuel discharge orifice placed substantially at the throat of a venturi formed by a constricted part of the intake passage and the auxiliary throttle member (UK Pat. No. 1 469 590 and U.S. Pat. No. 3,889,552 (Bauer)).
However, fuel flow control in such carburettors of conventional design is by a mechanical linkage between the auxiliary throttle and the movable member (generally a needle) of a variable jet. That construction has a number of disadvantages:
the mechanical coupling between the auxiliary throttle member, forming an air flow meter, and the variable jet involves a linkage whose initial adjustment is difficult and resistance to wear is limited, in which friction must be as low as possible;
there is generally no sufficient space available for locating an enrichment throttle valve upstream of the fuel delivery jet and consequently it is not possible to use a conventional starting device of the type used on carburettors of fixed geometry;
the annular passage defined by the jet and a movable needle has a wet surface much larger than that of a conventional fixed jet; the flow is consequently much more sensitive to the viscosity of the fuel, which makes the device very sensitive to temperature;
any lack of concentricity between the needle and the jet causes the equivalent hydraulic radius to vary considerably and consequently considerably affects the flow; if a guide is used for centring the needle, it increases the friction forces and complicates the construction.
It is an object of the invention to provide an improved carburation device in which the respective deficiencies of the two above kinds of carburettors are cured to a substantial extent and which is simple in construction and efficient in operation whatever the load and running speed of the engine.
For that purpose, the discharge orifice of a carburation device of the type set forth is located at the throat of the variable venturi, which is typically defined by a bulged portion of the intake passage wall and an auxiliary throttle formed as a streamlined oscillating vane. The solenoid valve is opened at least once per revolution of the engine for a variable time duration and preferably with a proper phase with respect to the dead centers of the engine pistons. Proper timing of fuel delivery once per turn may compensate for the inherent unbalance of fuel distribution between the engine combustion chambers in a 4-cylinder engine.
The solenoid valve will be of the type having a full opening condition and a zero or reduced opening condition and will assume one of the conditions at rest and the other during the period of each energizing electric pulse.